tv [untitled] September 8, 2010 12:30pm-1:00pm PST
that went from the wall, the rod, and then the block of concrete. if the wall tried to move, it would engage the rod, which would engage the block of concrete. these were built as gravity walls. they're very, very wide at the base. gosh, some of them are six, eight, 10 feet wide, and they are like dams. they are faced with basalt blocks that came from the quarries in marin county. they have performed very well. the classic gravity wall. there is enough friction along the base, and there is a slope on the backside, so there is enough weight pushing down on the wall that keeps it from moving. >> this is on broadway, along
rage and russian hill. >> that is a completely different design. this is truly what is called a cantilever retaining wall. it is designed to rotate and move. you can see the quality of construction. probably what they did in those days, they would take the rock that was present in the vicinity and mix it with the cement and create concrete. so there is no quality control. the strength of that wall various props -- varies from batch to batch, but it is safe. that is a gatt -- that is a gravity wall with a buttress in it. the diagonal strokes are buttressedes to keep the wall from moving. the wall is actually providing support to the house is above. you cannot tolerate any movement.
so they put these angles of concrete called buttresses. >> what would be under that? >> that is the cathedral at the corner of california and taylor. the rock under here is franciscan. it is sandstone. it is probably the hardest rock in san francisco at this location. we have just recently completed an investigation for the expansion of cathedral school, and they chose to go down 20 feet below the surface of the street, and it was incredibly hard. it was harder than the concrete that was placed as a foundation. the rock beneath the concrete was 10 times stronger than the concrete that is used to support the building. >> okay, moving to another part
of the city, this is the eastern side. >> this is on army street? >> yes, this is army st., se. look at all that stuff. it is an active city. >> a lot of the old industrial, the american can co., goodman lumber. all that good stuff. >> wow. >> and this building is one of the examples of remaining 1906 earthquake damage. it has been repaired above, and that is where they repaired the damage. what did they found these buildings on, back in the early days? >> those days, remember i mentioned early on, it would use of redwood grillage and they
would extend the grillage up far enough so it would spread out the load. today, one would know when to evaluate a building like this, we say to ourselves, there is no way this building can be standing. the bearing pressures that are being posed on the soil far exceed the strength of the materials present, yet it works. it could be such a phenomenon as arching and other things to keep the building standing. typically, today, we would support this building on pilings. we would support this going down to more confident material. within this is phil, bay mud, shall mud, and then down sand. re tom hill, at one time second street ended at howard. at that point, there was a large hill. the wealthy people of san
francisco lived on top of the hill, and churches were built along howard. then the city fathers in the 1920's let us knock down the hill, let us extend second street, and it finishes at the ballpark now. as you move south along second, that whole area, you get into rock. many of these old warehouse buildings, as you approach rincon hill, are sitting on rock. if you go down to king street, which is the street that fronts the ballpark, if you cross the street from the ballpark and the look on the sidewalk, there is actually a little brass plates that depict the location of the bluff that existed, the
transition from the beach to the vertical cliffs that existed at second street. at second and king. all of that was taken down, and they have built warehouses. now at his condominium developments. and portions of mission bay. the first building of mission bay, third and townsend -- i am sorry, third and king, it is a rock site. if you go further into mission bay, it goes from rock to week bay mud. -- weak bay mud. this is on the folsom? >> the old wonder bread bakery. a little bit higher up.
>> that particular building, if you look closely, the rock has actually receded back because of rockfalls. it was too expensive to put in foundations, so the foundation for that building is right on the surface. over the years, the rock face has receded to the point now where the flooding in the face of the bluff come together -- the footing and the face of the building and the bloc come together and are exposed. >> the confidence in the building is starting to be undermined. down at the bottom of the hill, out towards the bed, there are all kinds of uses there. what on earth would they be doing? >> what you are looking at here, two things occur along the south waterfront. one, sand that is dredged from
the sacramento river is actually put on barges and brought down into the bag at pier 80 and 92, and then they stockpiled the sand and wash it and tip it over to the concrete producers, bodhi, central, and others, rose and jensen, hanson, and the sand is coming down the sacramento river to mix with the aggregate to make concrete. the other thing that happens down in this area of san francisco, we are becoming more and more green at society. we are taking the asphalt and recycling it, and then we take concrete and put it through -- they will put it through a grinder and grind it to
different rock sizes and use that as the aggregate for asphalt or concrete. i am not sure if this is a sand pile for the concrete or if this is a recycle plant. but it is one of them. the city has opened up the large piers that unfortunately are not being used. we don't have the commerce we had in the old days. it has gone to oakland. we don't have the real site on the side of the bay. >> south of market has very unusual conditions, settlement. >> south of market, that particular fixture is at sixth street, which is right in the heart of this.
some of the largest magnitudes of movement during the 1906 earthquake occurred in this area of what is called sullivans march, which it sullivans march, just south of market. not only did it solidify and saddle, but when the ground slopes, and actually flows and moves latterly. some areas of mission creek and sullivan's marsh, that was on the order of 15 feet. it was significant. everything you see south of market was built after the 1906 earthquake. the longer the time between the 1906 earthquake and the time of construction, the more likely people forgot about all the
damage that occurred in 1906. so they were not as sensitive and perhaps cautious as they should be. many of these buildings that we see south of market, while they appear to be in excellent shape and are performing well, have not been either seismically strengthened or had at the subsurface conditions improved, so they are waiting. there could be significant damage during the next major earthquake. >> that reminds me of one of the central questions i wanted to ask you. which is this example relief focuses on the point. that is, what would you recommend for someone who is considering purchasing a building in the way of a consultation with geotechnical
engineers such as yourself, just to get a little heads up on what they might be facing in the lifetime of their ownership of the building? >> i believe it is in a prospective buyer's best interest, if they're buying an existing building and commercial area, that they consult with a structural engineer to make sure the building is structurally adequate and that to consult with a geotechnical engineer. dewitt least point out to them whether the building their purchasing -- to at least point out to them whether the building their purchasing is in a potential hazard zone. it not only has liquefaction, but it also has a hillside stability issues. while it may be stable sporadically, when the
earthquake comes, the slopes may fail. as a minimum, they should retain the services of a geotechnical and structural engineer, and there are well qualified people who come out. i am not just talking about and inspection service, i'm talking that licensed engineer, geotechnical, to look around, look at the maps available, and write an opinion letter as to the adequacy of the structure or the soil. there is another thing to consider when you are buying and building to the east of the van ness ave. a third map. this map depicts the area of the fire that followed the 1906 earthquake.
the blue is where all the buildings were destroyed or badly damaged by the fire. as a result, the ashes from those fires remained on site. in those days, lead paint was used to paint the sides of buildings. it is a known carcinogen. it is a hazardous material. what they did was after the earthquake, they took these mounds of sand, the valencia st. hill, if you go up market street, you know how it starts to climb? imagine that was a much larger hill. they mined 26 million cubic yards of sand out of theire, and they mixed it with the debris from the fire and they used it to fill basements. so you have sand, ash, pieces of
wood, brick. well, the ash is a hazardous material. you come along and used to buy a site at sixth and mission, with the understanding you will demolish the building and build a 50 story building. and you thought you were just going to dig out the sand, the film, and haul it to another site. now you come to find out the material you were going to move at $8 per ton is now going to cost anywhere from $40 to $78 per ton. the has a significant impact -- and has a significant impact on the cost of your project. if you are out on the sunset and vine a single-family home, just have the geotechnical engineer come out and say it is on earth quicksand, the building will
probably settle three-quarters of an inch. have the structural engineer look at the frame and say, you have a serious soft story here. lawrence, expected that. you might want to put in some clips and fasteners and sheer walls to stiffen it out. as you move east, you have not only the soft issues, the structural issues, but you have the hazardous-waste. if you recall as a kid, every intersection had for gas stations? now you cannot find a gas station anywhere? those four gas stations had leaky tanks. now we also have to look at the presence of hydrocarbons in the soil. >> moving along, the perfect setup for the next question. >> when we went out on our long
walk together, we talked about the relationship between the nature of a building, primarily its height, and the soil on which it sits. the slide is a good reminder of that. >> the ground has a natural period. the denser the material, the harder the material, the shorter the period of the ground. bay mud, which is present close to the shoreline, and the green area, the crown has a very long period. when the earthquake comes, there is a link of time that it takes the ground to go from one, slosh back and forth, measured in seconds. the deeper the bay mud deposits, the greater the length of time it takes for the motion to go one direction and back. so as you approach soft sites, loose sand sites, bay mud sites, the period of the site is long.
if you come along and build a tall building, and the ground is going like this, we know that tall buildings, the taller the building, the longer the period, the longer the time it takes to go from one side and back. iand the rule of thumb, every story that you add, it adds a 10th of a second to the period. if you have a 40 story building, you have a building that takes four seconds to go from one side to the other and back. now, that for second building is sitting on 100 feet of bay mud that also has a period of four seconds. the combination of those and harmony, what happens is they amplify the motion and the
building will collapse. example, the infamous bridge. in that case, the wind got in harmony with itself and it collapsed. what you try to do is you try to put a building on a site so that if the ground it is a long period, you want the building to be short. if the ground is short period, you want the building to be long. that is what happens when you get into the financial district. quite often, they will go down through the bay mud, put the structure on dense sand that has a very short period, and building is at 40 stories high with a very long period. even though the ground is doing this, the building is slowly moving. so there is damage, but there is less risk of damage. alternatively would have done is
layers and we've springs and all kinds of different -- and leaf springs and all kinds of different tools that can take the structure and put them on face. -- on phase. so when you look at a building like this, 10 stories, the period of that is about one second, 1.5 seconds. that is probably sitting on that sand or rock and that is about a quarter cent. so it is ideal. the ground is going like this, the building does not feel it. you feel it, but the building, it is not doing damage to the structure. >> the building next to it, let's say it is wood frame. what are thoseeperiods? >> there are less than a second. the beauty of a wood frame building, it is probably the most forgiving construction. it squeaks, the plaster will crack, and if you don't have the soft story to deal with, you
just have your cup of coffee and the one with your life. >> i guess this is turtle bill. >> grandview, turtle hill, san the mountain, -- sand mound, depending on how long you have been living there. >> this is northwest, of golden gate park. the nearest background, sunset and the richmond district? >> way out to the end, that is for smiley, the veterans hospital. this is for smiley. rock, rock, rock, rock, rock. here is hunters point. that rock is called serpent tonight. -- serpentinite. along this zone, there is another rock type, volcanic
rock. it is serpentinite. it is a very treacherous rock. if you go up market, you will see it where the met -- barack is exposed in that cut. -- for rock that is exposed in that it is cut. it can be hard work and have the consistency of soap and just slides. why is it like that? it is an intrusion. it is like toothpaste that was extruded through a crack in the earth. that means it was a sheer zone or fault, hundreds of millions of years ago, where it actually came up through and bubbled out and formed this band of serpentinite. adjacent to that is this wonderful rock. what is the other thin that is bad about serpentineite?
it contains naturally occurring asbestos, which is a carcinogen, known carcinogen. if you go to where st. mary's cathedral is, cathedral hill, that is one of the outcrops. there is a high percentage of serpentinite in the rock. in the old days, the 1930's, 1920's, san franciscans to not bother to deal with the disposal companies. they would take their garbage out the other side of twin peaks, bump their garbage, and shovel sand on top of it. so in the old days, you could go out there and build your home and be surprised that you would find sanitary landfill or construction debris landfills. they were prevalent out there. >> this is twin peaks? >> going up to twin peaks.
>> one form does not change much. >> eucalyptus trees have got much taller. that is not halt leading up to predict that is the hill leading up to there. fifth avenue, that area of san francisco. ucsf. that is this area right here. >> it is an earthquake hazard micro zone. >> again, another example of the wpa. the streets, they wanted to get to the top of the hill, said the duties beautiful walls and stairways. they are doing well. >> all of these hillsides have catalyzed the design and construction of the interesting foundations and sub framing. >> up there, you'll find quite
often, because it is rock, they have dug holes, filled it with concrete, put pulls up, and built their buildings. they have been there actually now 67 years. >> some of these are significant earthquake hazard? >> many of them, because they're not properly braced, because the foundations are not deep enough. the consequences could be significant during an earthquake. >> the tunnel opened that whole section up to development. did that lead to park side? >> west portal is right here. it this slope. westport open up this whole area down here, with the reservoir. the tunnel was built and two different ways. one of them was cut and cover, the other was a tunneling machine. as they came close to the side
of the hill, they dug down vertically, took out the dirt. they took the dart -- they took the dirt and replaced it as phil -- fill all along the lower part of market street. they filled in the ravine. >> thank you all very much. that was a heck of a tort of san francisco. thank you for your attendance. we hope to see you every third thursday right here.